The inkjet recording has been known as an advantageous system as it can easily attain full-colored images with a simpler process than that of other recording systems, and also attains images of high resolutions by apparatus of simple configuration.
For inkjet recording, dye inks in which various water-soluble dyes are dissolved in water or a mixed solution of water and an inorganic solvent are used as inkjet recording inks. These dye inks have excellent clear color tone, but has poor light resistance. To compare with the dye inks, pigment inks in which a carbon black or various organic pigments are disposed have excellent light resistance, and thus researches have been actively conducted on such pigment inks.
Since pigments are insoluble to water unlike dyes, it is important to stably disperse the pigment in water in the state of fine particles. However, such dispersion state is not necessary easy to achieve. Especially when the thermal condition for the dispersion system of a pigment is changed, absorption equilibrium of a dispersant and the pigment is disturbed, affecting the interaction between the pigment particles. As a result, the ink may cause the physical change and/or a large amount of the aggregated matter may be generated during the storage of a long period. The physical change, especially a change in its viscosity, and/or generation of aggregated matters can be crucial to an ink for use in an inkjet printer. This is because, the physical change and/or generation of aggregated matters become factors to cause the change in the properties of a head and/or blockage of an ejection nozzle, and thus there is a case where a proper printing cannot be performed. In addition to this, although it is desired to reduce a particle diameter of an ink for the purpose of improving preciseness of images, the pigment ink of a small particle diameter tends to cause aggregation of particles depending on the selection of a system of disperser, dispersing process, and the like, and thus stable ejection cannot be always ensured. As has been mentioned above, there has not been provided any inkjet recording ink which solves all of the above-mentioned problems.
In order to prevent a nozzle blockage or improve a color gamut, it is necessary to reduce an average particle diameter of a pigment for use. However, in any ink containing a polymer dispersant, such as styrene-acryl copolymer or styrene-maleic acid copolymer (see Patent Literature 1), and a sodium naphthalenesulfonate-formalin condensate (see Patent Literature 2) or containing a surfactant, such as polyethylene glycol alkyl phenyl ether (see Patent Literature 3), polyethylene glycol alkyl phenyl ether sulfate (see Patent Literature 4), and polyethylene glycol alkyl phenyl ether phosphate (see Patent Literature 5), an average particle diameter of a pigment is large, and thus these inks have poor ejection stability or liquid stability.
Moreover, it has already been proposed to add resin emulsion to an aqueous pigment ink, mainly for the purpose of improving storage stability. In addition to this proposal, as a proposal for preventing a change in physical property and/or generation of aggregated matters, for example, Patent Literature 6 discloses a method for removing aggregated matters that are generated during the storage for a long period in advance by heating a pigment ink at 50° C. for 100 hours to 500 hours. Moreover, the technology disclosed in Patent Literature 7 realizes this treatment within a short period by heating a pigment, insoluble resin emulsion and saccharide at 65° C. to 80° C. for 0.5 hours to 3 hours. Patent Literature 8 discloses that an ink having a less viscosity change after storage for a long period can be provided by heating a pigment, water and polysaccharide in a solution having pH of 8 or more at 60° C. to 180° C. Patent Literature 9 discloses that an ink having a sharp particle size distribution, excellent glossiness and excellent color reproducibility can be provided by heating a colored fine particle dispersed ink containing a colorant and a resin at 35° C. or more. Patent Literature 10 discloses that an inkjet ink having excellent ejection stability, sufficient fixing ability and sufficient water resistance by defining an average particle diameter and glass transition temperature of a urethane resin. Patent Literature 11 discloses that an image fixing ability, ink storage stability, and ejection stability can be maintained at a desirable degree by selecting a certain polyurethane resin even when a particle diameter of a pigment is in the range of 70 nm to 180 nm. Patent Literature 12 discloses that a sufficient image fixing ability and ink storage ability can be attained by specifying a ratio of a urethane resin and a pigment, formulated amount thereof and a particle diameter of the pigment. Patent Literature 13 discloses that a sufficient image fixing ability can be attained by defining an acid value of a free acid of a urethane resin and a surface tension of an ink. Patent Literature 14 discloses that an image fixing ability, ink storage stability, and ejection stability can be attained with an ink containing resin emulsion in which a resin has urethane bonds by defining a primary particle diameter and DBP oil absorption of a carbon black.
However, the aforementioned polyurethane resin tends to aggregate with influence of an organic solvent contained in an inkjet recording ink, and there are some cases where image fixing ability, ink storage stability and ejection stability are lowered. There have not yet been provided resin emulsion which can attain completely desirable these properties. Especially, a polyurethane resin suitable for pigment inks containing relatively fine pigment having a particle diameter of approximately 100 nm, surfactant-contained dispersing pigment inks and acid carbon black pigment inks has not yet been provided. Moreover, the aforementioned conventional techniques do not mention about attachments of the ink on a head, and conventional surfactant-contained inks have not solved the problem on the attachments of the ink to an inkjet head having an ink-repellent layer containing a silicone resin.
It has been known that the effective method for attaining high quality images on normal paper is to reduce a surface tension of an ink so as to increase wettability of the ink to the normal paper, thereby improving coloring performance. In addition, it is effective to add a resin to the ink so as to improve a resistance of images. For example, Patent Literature 15 proposes an ink containing fine polymer particles, a water insoluble or poor soluble colorant, a water soluble organic solvent and a fluorine-contained surfactant. However, with use of such ink of low surface tension containing the fluorine-contained surfactant, a nozzle plate tends to be easily wet, and thus it is difficult ensure ejection stability.
Moreover, Patent Literature 16 proposes an ink containing a water soluble solvent, a colorant, water, and a polymer containing an alkyleneoxide unit, aromatic ring unit, and carboxyl unit at least in a single molecule, and discloses “liquid-repellent treatment for an area including and surrounding an ejection opening,” and “containing a silicone surfactant, fluorine surfactant, or the like” in the specification thereof. However, this literature does not disclose any specific types of the silicone surfactant, and it is difficult to ensure liquid repellency with the low surface tension ink containing a fluorine-contained surfactant with the structure of this proposal.
Moreover, Patent Literature 17 proposes an aqueous dispersion for inkjet recording containing a self-dispersing pigment, polymer particles and a compound having a polysiloxane skeleton, and an aqueous ink containing the aqueous dispersion. However, this ink has a surface tension of 25 mN/m to 50 mN/m, and thus it has not been intended to be as a low-surface-tension ink and no special treatment is not applied to a nozzle plate.
Therefore, Patent Literature 18 proposes to reduce a surface tension of an inkjet ink so as to increase wettablity of the ink to a recording medium, thereby improving an image quality, and also proposes to add a silicone resin to an ink-repellent layer of a nozzle plate placed in an inkjet nozzle. Although an image quality is improved by this proposal of the ink, the performance of the ink-repellent layer formed of a silicone resin is not sufficient to the conventional ink as it is required to have excellent physical resistance durable to wiping or the like, not only ink repellency at the initial stage, to comply to the recent trends, such as a high-speed printer or more frequent use. Accordingly, further improvement or development has been demanded.
Patent Literature 19 proposes an inkjet recording ink containing a carbon black, a dispersant, a urethane resin and water in which the urethane resin has a glass transition temperature of 50° C. to 150° C., and a particle diameter D50 of 5 nm to 20 nm. However, there is no description about the relationship between A (a particle diameter D90 of particles present in dispersion containing the carbon black, the dispersant and water, before added with the resin emulsion) and B (a particle diameter D90 of particles present in the inkjet recording ink), i.e. B−A, in this literature. In addition, the examples disclosed in Patent Literature 19 use color pigments, not a carbon black.    [Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 56-147863    [Patent Literature 2] JP-A No. 61-083267    [Patent Literature 3] JP-A No. 05-105837    [Patent Literature 4] JP-A No. 10-168367    [Patent Literature 5] JP-A No. 10-88050    [Patent Literature 6] JP-A No. 03-64376    [Patent Literature 7] JP-A No. 08-73785    [Patent Literature 8] JP-A No. 2002-30243    [Patent Literature 9] JP-A No. 2003-313475    [Patent Literature 10] JP-A No. 2007-84784    [Patent Literature 11] JP-A No. 2004-169008    [Patent Literature 12] JP-A No. 2004-285344    [Patent Literature 13] JP-A No. 2002-167536    [Patent Literature 14] JP-A No. 2002-97390    [Patent Literature 15] JP-A No. 2003-226827    [Patent Literature 16] JP-A No. 2004-10733    [Patent Literature 17] JP-A No. 2007-154021    [Patent Literature 18] JP-A No. 2005-138383    [Patent Literature 19] JP-A No. 2006-335858